DELAY OF THE WAVE. 31 



ance there, seven more hours are necessary for it to 

 get through the English Channel. The Xorth Sea 

 receives its tides from the two branches of the tide- 

 wave, one of which comes through the Channel, while 

 the other passes round Scotland. The same tide 

 which appears at Brest at noon, reaches Dover 

 and Calais about seven o' clock, and Ostend about 

 eight in the evening. The same tide running round 

 Scotland, arrives at the mouth of the Thames at 

 eight o' clock on the following morning, as well as 

 on the coast of Germany, where it meets and swells 

 the other wave that came up the Channel. 



The tide-water, rising in front of a river's mouth, 

 partly pours itself into the river, and partly 

 prevents the escape of its waters to the sea, so 

 that, in great rivers, the flood is felt many miles 

 up the stream, being however more and more re- 

 tarded the farther it advances ; so much so, that 

 it may easily happen, that at the river's mouth the 

 ebb may have already commenced, while higher up 

 the flood may still be rising. 



The causes however of these several delays of 

 the tide remain always the same ; the tides must 

 therefore ever follow each other in regular and equal 

 periods. Hence the times of their recurrence may 

 be calculated, from the position of the moon, any 

 length of time beforehand. The regular delay of 

 high water at any place, after the moon's passage of 

 its meridian, on the days of new and of full moon, 



